Congeneric, close-boiling mixtures of aromatic compounds cannot always be separated into their components by distillation. Usually their solubility characteristics are so closely related that selective extraction of the components is not feasible. Selective crystallization from special solvents usually requires one or more recrystallization steps to reduce the concentration of contaminants to an acceptable level, and solvent washing requires large amounts of solvent. Also, these two procedures are relatively expensive because recoveries of the desired product are generally fairly low and the cost of solvent purification, and/or discarding of contaminated solvent, is high. Generally conversion of contaminants to derivatives which are readily separated from the final mixture is not feasible because the desired product also tends to react in a manner similar to the contaminants. In fact, the source of the contaminants in the congeneric mixture is usually a chemical reaction which cannot be controlled with the degree of specificity needed to obtain an end product of desired purity.